The present invention relates to a laser apparatus, which emits a laser beam, and a method for controlling the same, and more particularly, to an exposure apparatus and an exposure method used during a lithography process for fabricating microdevices, such as semiconductor devices, liquid crystal display devices, charge-coupled devices (CCDs), and thin film magnetic heads.
An exposure apparatus is used when fabricating semiconductor devices to transcribe a pattern formed on a reticle (mask) to a wafer (substrate), to which a photosensitive material such as photoresist is applied. There is a demand for such exposure apparatus to have a higher resolution to miniature circuit patterns. To satisfy such demand, an excimer laser apparatus is used a's a light source to lengthen the short waves of the exposure light.
Such excimer laser apparatus performs an adjustment pulse emission in which pulses of laser beam are emitted cyclically or continuously without the operator's command during a ready state, in which the irradiation of a reticle or wafer by a laser beam is stopped. This maintains the gas state (density distribution) in the laser chamber. Further, control data to be needed for the emission of the laser pulse after the ready state, such as the relationship between the applied voltage and the output laser pulse energy, is acquired during the adjustment pulse emission.
The conditions of the adjustment pulse emission during the ready state, that is, the target energy of each pulse, the applying voltage corresponding to the target energy, and the oscillation frequency (emission period), are determined by the excimer laser apparatus itself and substantially maintained constant.
Due to the recent various exposure conditions, the emission conditions of the laser beam during exposure are often changed in a complicated manner. Further, the range of the emission condition changes have become wide.
However, the laser's ready state emission condition may differ greatly from the laser's exposure state emission condition since the emission condition of the laser beam during the ready state does not take into consideration the emission condition for the subsequent exposure.
The excimer laser apparatus is also easily affected by a slight change in the target energy or the oscillation frequency. Thus, when the laser beam emission condition in the ready state differs greatly from that of the exposure state, the laser beam output may become unstable if the laser beam output control during exposure is performed based on control data determined in the ready state. Especially in the beginning of the laser beam emission during exposure, the required laser beam energy and the actual emitted laser beam energy may differ greatly.